The present invention relates to an improvement in a passive infrared intrusion detector or detection system for measuring an infrared ray emitted from an object so as to detect the intrusion of an object into a controlled or monitored zone.
A conventional passive infrared intrusion detector detects changes in a level of infrared rays in a target supervising area by using a passive infrared sensor for sensing infrared energy emitted from an object, thereby detecting the intrusion of the object.
FIG. 1 is a block diagram of a conventional passive infrared intrusion detector. Referring to FIG. 1, reference numeral 1 denotes a multi-faceted parabolic reflector. The reflector 1 provides optically divided detection zones 1.sub.1 to 1.sub.n. Reference numeral 2 denotes a pyroelectric infrared intrusion sensor or detector as a passive infrared sensor. The pyroelectric sensor 2 receives infrared rays focused by the multi-faceted parabolic reflector 1 from the detection zones and detects changes in the level of infrared rays. The pyroelectric sensor 2 generates an output signal corresponding to these changes. Reference numeral 3 denotes an amplifier for amplifying the output signal supplied from the pyroelectric sensor 2 through a coupling capacitor 4, so that an amplified signal is suitable for the following signal processing. Reference numeral 5 denotes a filter for extracting only a signal within the range of 0.1 to 10 Hz from the signal supplied from the amplifier 3 through a coupling capacitor 6. The signal range of 0.1 to 10 Hz corresponds to a moving speed of an object (e.g., man) within the zones 1.sub.1 to 1.sub.n. Reference numeral 7 denotes a level comparator for comparing an absolute value of the signal supplied from the filter 5 with a reference value Vr generated from a reference voltage generator 8. When the absolute value of the output signal from the filter 5 exceeds the reference value Vr, the level comparator 7 supplies an output to an alarm output circuit 9. The alarm output circuit 9 then generates an alarm signal.
In the passive infrared intrusion detector having the arrangement described above, when an intruder (i.e., object or man) crosses any one of the zones 1.sub.1 to 1.sub.n of the multi-faceted parabolic reflector 1, this intruder serves as a chopper. More particularly, when the intruder enters any one of the zones 1.sub.1 to 1.sub.n, the infrared energy focused by the parabolic reflector 1 and sensed by the pyroelectric sensor 2 changes in level. A change in level of infrared energy is represented by a change in level of the output signal generated from the pyroelectric sensor 2. In this manner, the output signal from the pyroelectric sensor 2 is amplified by the amplifier 3, and the amplified signal is filtered through the filter 5. The filter 5 thus extracts a signal which falls within the frequency range of 0.1 to 10 Hz and which corresponds to a moving speed of the intruder. Noise signal components caused by movement of objects excluding the intruder can thus be eliminated. The absolute value of the output signal from the filter 5 is compared by the level comparator 7 with the reference value Vr generated from the reference voltage generator 8. When the absolute value of the output signal generated from the filter 5 exceeds the reference value Vr, the level comparator 7 generates a signal which represents the presence of an intruder. This signal is supplied to the alarm output circuit 9 which then generates the alarm signal.
However, in the conventional passive infrared intrusion detector having the arrangement and operation mode as described above, external noise and one-shot popcorn noise generated by the pyroelectric sensor may cause a malfunction and erroneously generate an alarm signal.